1. Field of the Invention
The present invention relates to a self-dispersible colorant by incorporating hydrophilic groups into a colorant, a method of preparing the colorant and to an ink composition comprising the colorant. More specifically, the invention is directed to a self-dispersible colorant that has excellent dispersibility in aqueous solvents even without the use of other dispersants. The invention is also directed to a method of preparing the colorant and an ink composition comprising the colorant.
2. Description of the Related Art
In general, colorants embody their inherent colors by selectively absorbing or reflecting visible light, and are classified as dyestuffs and pigments.
Dyestuffs which are dissolved in solvents, are dyed in any materials to be dyed, such as fibers, leathers, furs, papers, etc., thereby providing fastness to daylight, washing, friction, etc. Pigments which are insoluble to solvents, are coloring matters in the form of particulates, and are not directly dyed on the surface of the material to be dyed but are adhered to the surface of the material to be dyed, by physical means (e.g., adhesion, etc.), thereby providing their inherent colors.
In preparing pigment-type inks, an operation of dispersing a pigment in a solvent should be basically carried out. Herein, the size of pigment particles, the distribution of its particle size and its dispersion stability are important factors influencing on efficient dispersion of the pigment. Even if the pigment particles dispersed through the dispersing step described above are stored for long term or are exposed to the change of environmental factors such as change of temperature, aggregation or precipitation of the pigment particles should not occur.
Most of pigment-type black ink uses a carbon black as a colorant. A method for dispersing a carbon black in an aqueous solvent for ink using a dispersant includes a dispersing method using a styrene-maleic acid anhydride copolymer as disclosed in U.S. Pat. No. 3,687,887. Alternative dispersing methods use a hydrophobic polymer dispersant containing a hydrophilic group such as a carboxylic acid, a sulfonic acid or a sulfate as disclosed in U.S. Pat. No. 4,697,794. A random copolymer that contains a hydrophilic group by incorporating a carboxylic acid is used as a dispersant as disclosed in U.S. Pat. Nos. 5,229,786; 5,172,133; 5,160,370; 5,184,148; or 5,106,417, as well as others. Also, U.S. Pat. Nos. 5,085,698 and 5,221,334 disclose an AB-type block copolymer or a BAB-type block copolymer (herein, A is a hydrophilic monomer and B is a hydrophobic monomer) in which a hydrophilic monomer and a hydrophobic monomer are used, as a dispersant, and U.S. Pat. No. 5,589,522 used a graft polymer and U.S. Pat. Nos. 4,959,661 and 5,125,968 disclose a known emulsifier to enhance the dispersibility of a pigment.
When using a dispersant in order to disperse a carbon black, the dispersant is easily available since conventional dispersants can be used. However, when using a water-soluble dispersant in order to disperse a carbon black pigment in an aqueous solvent, the water-soluble dispersant can be adsorbed physically to the surfaces of carbon black. The dispersing effect of the pigment can be obtained only with an excessive amount of the dispersant. Thus, since an excessive water-soluble dispersant must be used compared to the amount of the pigment to be dispersed, overall dispersion efficiency is decreased. Since the dispersant does not bind to the pigment efficiently and stably, the dispersion stability becomes poor. Thus, when preparing ink by using the dispersant, long-term storage stability is decreased such that insoluble particles in the ink are prone to aggregation or precipitation.
When a block copolymer and a graft copolymer are used as a dispersant, the cost for preparing the dispersant is high. When using a general emulsifier, excessive foam may occur during the dispersion process.
To solve the problem occurred by use of such a dispersant and an emulsifier, a method of enhancing the dispersibility of a carbon black by surface modification was designed. U.S. Pat. Nos. 5,630,868 and 5,672,198 disclose a method of modifying a carbon black pigment to a self-dispersible pigment by incorporating a hydrophilic group into the surface of the carbon black through a reaction of the carbon black pigment with a diazonium salt.
However, according to the method disclosed in these patents, an aromatic amine is necessarily required in the terminal end of a compound that contains a hydrophilic group and reacts with a pigment to transport the hydrophilic group to the pigment. However, such hydrophilic group-containing aromatic amine compounds are difficult to obtain commercially, and the aromatic amine compound should be reacted with a carbon black after converting the compound to a diazonium salt. Thus, a plurality of reaction processes, not a single process, are required and so the complexity of the process and the cost of the process are increased.